It is well-known that rotating waves of electrical activity are a factor in potentially dangerous cardiac arrhythmias such as ventricular tachycardias and ventricular fibrillations (“ventricular tachycardia events”). The rotating waves, or reentries, that are responsible for ventricular tachycardia events are classified into two categories: 1) functional reentries, which involve freely rotating waves; and 2) anatomical reentries, where a wave rotates around an obstacle such as a blood vessel or piece of ischemic tissue. The latter are referred to as being ‘pinned’ by the obstacle. Traditional defibrillation is not a preferred way of dealing with such rotating waves because defibrillation resets electrical activity everywhere in the heart and uses high voltage shocks, which have undesirable side effects.
One common method of attempting to terminate these rotating waves or reentries is antitachycardia pacing (ATP). ATP has a high rate of success in dealing with functional reentries, but is not as effective against anatomical reentries. Generally, if ATP is not effective, a defibrillating shock of large amplitude is applied directly to cardiac muscle.
The reasons ATP is not always successful can be found in the complexity of the system. In one-dimension, the situation involving reentries is well-understood and relatively simple. A reentry essentially consists of a pulse rotating along a closed ring. To terminate a reentry, it is enough to deliver a stimulus close to the tail of the rotating pulse. The stimulus should be delivered inside the critical time interval (the so-called vulnerable window, VW). Under these circumstances, only one pulse is created and it propagates in the direction opposite to the reentry pulse. Ultimately, it collides with the reentry pulse, leading to complete annihilation. If an ATP stimulus is delivered to quiescent tissue, it creates two counter propagating pulses and the reentry is not terminated. The description above assumes, however, that the medium is homogeneous.
The situation is more complex in two dimensions. In this case, an anatomical reentry is a spiral wave rotating around an obstacle. To terminate the reentry, it is necessary to create a wave that can annihilate the rotating wave. This is more difficult than in the one-dimensional scenario because, in the two-dimensional scenario, the wave is characterized not only by its sense of rotation, but also by a complete wave form. As in the one-dimensional situation, an ATP stimulus must be delivered inside the vulnerable window, but this condition alone is not sufficient. When the ATP electrode is situated far from the obstacle, the nucleated wave has a free end that is separated from the obstacle. Thus, ATP is only successful when the free end merges with the obstacle. Only then, two counter propagating waves annihilate and the anatomical reentry is terminated. This is possible only if the distance from the free end of the nucleated wave to the obstacle is smaller than a critical distance (of the order of the core size of a free vortex, from several cm to several mm in cardiac muscle).
On the other hand, when a stimulating electrode is placed far from the obstacle, then ATP does not terminate the reentry, but instead creates a free reentry in addition to the anatomical one. When ATP is not successful, it is usually followed by conventional defibrillation techniques, which have unwanted side effects. These effects may include: (1) transient ectopy, tachycardia or induction of ventricular fibrillation; (2) depression of electrical and mechanical functions; (3) bradycardia, complete heart block and increased pacing thresholds; (4) atrial and ventricular mechanical dysfunction (stunning), which is directly related to the strength of shocks; (5) significant elevation of Troponin I serum level in patients after spontaneous cardioverter defibrillator shocks; (6) decrease of the myocardial lactate extraction rate by mitochondria. In addition to physical damage to the heart muscle, defibrillation therapy is also associated with psychological side effects. High energy discharge of a defibrillator in a conscious patient is painful and extremely unpleasant. Recent clinical studies have demonstrated that ICD patients have a significantly higher incidence of anxiety, depression, and panic disorders than do the general population.
What is needed, therefore, is a method for terminating an anatomical reentry using an energy level lower than that of conventional defibrillation techniques. Further, it is desirable that such a method be effective even when the precise location of the reentry is unknown.